The present invention relates to a magnetic recording medium, more specifically, the present invention relates to a magnetic recording medium for high-density recording, having a thin magnetic layer and having very excellent electromagnetic conversion characteristics and running durability.
The magnetic recording medium widely used as a magnetic recording medium for computers (e.g., disc, tape) is obtained by providing a magnetic layer comprising a binder having dispersed therein ferromagnetic powder particles on a nonmagnetic support. Recently, in the field of magnetic recording, digital recording is practically taking the place of conventional analog recording because of its less deterioration of recording. In the digital recording, a large number of signals must be generally recorded and moreover, the recording/reproducing apparatus and the recording medium used therefor are demanded to ensure high image quality and high sound quality and at the same time, realize downsizing and space saving. To cope with these requirements, still higher density recording is demanded.
For achieving higher density recording, recording signal with a short wavelength and recording locus in a narrow track are employed. As for the recording medium, more improvement in the electromagnetic conversion characteristics is demanded and for this purpose, various proposals have been proposed, such as improvement in the magnetic characteristics of ferromagnetic powder, formation of fine particles, high filling, or ultra-smoothing of the medium surface. These are, however, not satisfactory for the higher density recording demanded in recent years.
When the recording wavelength is shortened, if the magnetic layer thickness is large, the self demagnetization loss at the time of recording and the thickness loss at the time of reproduction come out to serious problems. In order to avoid the saturation of reproducing head, the magnetic layer is rendered thinner, however, if the thickness of the magnetic layer is reduced to about 0.1 xcexcm or less, the effect of nonmagnetic support is liable to appear on the magnetic layer surface and the reduction of electromagnetic characteristics and the error rate are liable to increase. Therefore, it is important to reduce the size of the magnetic powder so as to reduce noises and improve the electromagnetic conversion characteristics. For this purpose, a ferromagnetic hexagonal ferrite fine powder having a tabular diameter (plate size) of 40 nm or less is being used. However, such a fine particulate ferromagnetic powder readily undergoes aggregation and therefore, good dispersion of the powder is very difficult to attain as compared with conventional ferromagnetic fine powder having a relatively large size.
When the thickness of the magnetic layer is reduced, the running durability becomes a problem. JP-A-11-86273 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) describes a magnetic recording medium where diamond particles having an average particle size of 0.10 to 1.0 xcexcm are contained in the magnetic layer in an amount of 0.01 to 5% by weight (mass %) based on the ferromagnetic powder and thereby, high electromagnetic conversion characteristics and high durability both are attained. However, when use of an MR head becomes inevitable with the progress of higher density recording, the above-described method is disadvantageous in that diamond in the magnetic layer falls off and accelerates the abrasion of MR head.
The object of the present invention is to provide a magnetic recording medium optimal for digital recording, which has good dispersion of magnetic powder, excellent electromagnetic conversion characteristics and high running durability.
As a result of extensive investigations to achieve the above-described object, the present inventors have found that when a specific amount of fine diamond particles are used as an abrasive, a ferromagnetic hexagonal ferrite powder having a specific tabular diameter is used as the magnetic particle and a binder containing a specific polar group in a predetermined amount is used, the dispersibility of fine particulate magnetic powder is improved and a magnetic recording medium having excellent electromagnetic conversion characteristics and high running durability can be obtained. The present invention has been accomplished based on this finding.
The object of the present invention can be attained by a magnetic recording medium having the following constitution.
1. A magnetic recording medium comprising a nonmagnetic support having thereon at least one magnetic layer containing a ferromagnetic hexagonal ferrite powder and a binder, wherein
(1) the magnetic layer contains diamond particles having an average particle size of 0.03 to 0.5 xcexcm in a ratio of 0.1 to 5% by weight to the ferromagnetic hexagonal ferrite powder,
(2) the ferrite hexagonal ferrite powder contained in the magnetic layer has an average tabular diameter of 5 to 40 nm, and
(3) the binder contained in the magnetic layer is at least either one of:
(i) a binder containing from 0.2 to 0.7 meq/g of at least one polar group selected from xe2x80x94SO3M, xe2x80x94OSO3M, xe2x80x94PO(OM)2, xe2x80x94OPO(OM)2 and xe2x80x94COOM (wherein M represents a hydrogen atom, an alkali metal or ammonium), and
(ii) a binder containing from 0.5 to 5 meq/g of at least one polar group selected from xe2x80x94CONR1R2, xe2x80x94NR1R2 and xe2x80x94N+R1R2R3 (wherein R1, R2 and R3 each independently represents a hydrogen atom or an alkyl group).
Preferred embodiments of the present invention are as follows.
2. The magnetic recording medium as described in 1 above, wherein the binder is Polyurethane Resin (A) obtained by reacting an organic diisocyanate, a polyol having a cyclic structure and an alkylene oxide chain and having a molecular weight of 500 to 5,000, and as a chain extender, a polyol having a cyclic structure and having a molecular weight of 200 to 500.
3. The magnetic recording medium as described in 1 above, wherein the binder is Polyurethane Resin (B) obtained by reacting an organic diisocyanate compound, a polyester polyol comprising an aliphatic dibasic acid and an aliphatic diol having no cyclic structure and having an alkyl branched side chain, and as a chain extender, an aliphatic diol having a branched alkyl side chain having 3 or more carbon atoms.
4. The magnetic recording medium as described in 1 above, wherein the binder is Polyurethane Resin (C) obtained by reacting an organic diisocyanate and a polyol compound having a cyclic structure and an alkyl chain having 2 or more carbon atoms.
5. The magnetic recording medium as described in 1 above, wherein the binder is a vinyl chloride-base copolymer containing at least 75 to 95% by weight of a vinyl chloride monomer unit.
6. The magnetic recording medium as described above, wherein the binder is an acrylic copolymer comprising from 1 to 75% by weight of a nitrogen-containing radical polymerizable monomer unit, from 1 to 75% by weight of an aromatic ring-containing radical polymerizable monomer unit, and another radical polymerizable monomer unit in an amount of making a total of 100% by weight.
7. The magnetic recording medium as descried in any one of 1 to 6 above, which is used for MR head reproduction.
For attaining a high capacity of a magnetic recording medium, the increase of track density is indispensable and the narrow track width requires use of an MR head. The MR head is high sensitive but susceptible to medium noises. The noises are ascribable to the non-uniformity in the dispersion state of magnetic powder. In order to reduce the non-uniformity in the dispersion state of magnetic powder, it is effective to perform the dispersion using a binder containing the above-described polar group in the above-described amount as in the present invention and use diamond capable of greatly improving the magnetic layer strength by the addition in a small amount.
Conventional binders induce the falling of diamond and this gives rise to deterioration of durability and reduction of output due to abrasion of the MR head. In the present invention, diamond having a specific size is dispersed in a specific amount using a binder having a specific polar group and this is effective to prevent the falling of diamond, improve the durability and inhibit the abrasion of head.